In the oil industry the use of the seismic method of geophysical exploration has generally involved the use of multiple individual independent amplifier channels connected to separate two conductor cables to a geophone unit, which might include one or a plurality of geophones. The amplifiers are designed to be as free of amplitude and phase distortion as possible, since the record of the geophone signal involves the precise measurement of amplitude at discrete digitizing times, to a precision of 12-16 bits, or more.
Noise and distortion can modify the geophone analog signal during its transmission from the geophone to the amplifier. Thus special attention has been devoted to the design of the cables, and individual conductor pairs have been provided for each amplifier channel, to minimize cross talk, noise and distortion.
Also, it has become quite common to use seismic line amplifiers, positioned at the geophones to linearly amplify the geophone analog signals before transmission over the cable, so as to have a stronger signal and to minimize noise distortion.
More recently a new system of seismic data acquistion has been perfected (see U.S. Pat. Nos. 3,881,166 and 3,883,725) in which the geophone signals, in contrast with conventional practice of digitizing to 12-16 bits, are digitized only to one bit. The amplification and digitization are done in each of a plurality of array terminals, which transmit the 1 bit digital signals as trains of single bits along a single two conductor cable which passes serially through each of the array terminals.
The geophone signals which are amplified and digitized to one bit in the array terminals, can be transmitted as square wave one bit digital signals on a time shared basis, for all of the array terminals, over a single pair of conductors, with a minimum of distortion and error.
The use of this transmission circuit for the major part of the distance from the geophones to the recording truck, minimizes the cable problem, to the extent that the individual analog geophone signals are only transmitted over short lengths of cables, which are about 1/Nth of the conventional length, where N equals the number of array terminals.
However, as of prior to this invention, there is still analog signal transmission over individual cables, which can still cause noise, and phase and amplitude distortion. It is customary also to include high fidelity seismic line amplifiers at the geophones to minimize distortion in the signals that reach the array terminals, all of this at greater expense in time and money.
It is therefor a primary object of this invention to simplify the transmission of geophone signals to the array terminal, and make the signals less sensitive to noise and phase distortion in the cables.
It is a further object of this invention to provide a system for transmission of geophone signals to array terminals whereby all geophone signals going to a single array terminal can be carried on a single pair of conductors.